Add a hidden preference to allow customizing support for URI scheme

Some add-ons provide support for other URI schemes for Firefox.
For example,
geo: URI scheme handler, lp:
URI scheme handler, and BBSFox.
We have some hidden preference:
extensions.scriptish.unmhtIsGreaseable and
extensions.scriptish.fileIsGreaseable. Can you add a hidden
preference to allow customizing support for URI scheme?

For example, a string type preference is named
extensions.scriptish.schemeGreaseable. Users can set value: lp,
geo, telnet.